Process of crystal formation



Jan. 2l; 1947. l c. D. wEsr 2,414,679

l PROCESS OF CRYSTAL FORMATION l led sept. 14, i944 FlG. 2

AZVENTOR.'

atentecl Jan. 21,1947

PROCESS F CRYSTAL FORMATION Cutler D. West, Cambridge, Mass., assigner to Polaroid Corporation, Cambridge, Mass., a corporation of Delaware Application September 14, 1944, Serial No. 554,003

11 Claims.

l This invention relates to crystallography and more particularly to a new process for producing single crystals of predetermined size and orientation. This application is a continuation-in-part at infinity comprising a series of concentric rings, but said rings will be of diierent sizes depending upon which side of the crystal is nearest the observer.

of my copending joint application with Freder- 5 It is accordingly a primary objective of the ick J. Binda, Serial No. 547,263, filed July 29, 1944. present invention to provide means for prevent- In said application there is disclosed a process ing the above effect, and more particularly, t0 for growing relatively large single crystals of provide an improved process for growing basal uniaxial material and particularly basal sections sections of uniaxial crystals wherein the optic of uniaxial crystals such as sodium nitrate. l In axis will be uniformly disposed throughut Subgeneral, said process comprises forming a melt stantially the entire crystal. comprising the desired crystalline material, Other objects and advantages will in part apbringing into contact with said melt a single Dear and in part be pointed out in the course of crystal of a material substantially infusible at the following detailed description of one or more the temperature of the melt, substantially insolembodiments of the invention, which are given uble in said melt, separable from a formed crysas non-limiting examples, in connection with the tal and different than said melt, said secondn accompanying drawing in which: named crystal having a plane surface thereof in Figure lois a diagrammatic view comprising a contact with said melt and having said surface vertical section through a mold used in the pracextending over at least a greater part of the tice of the invention, and cross-sectional area of the melt, said surface Figure 2 iS a diagrammatic Sectional View of a substantially defining the desired cross-sectional Crystal groWll by means of the apparatus and area of the said crystal to be formed, said crysprocess illustrated in Fig. 1. tal surface having an atomic 'structure capable AS has already been indicated, a urliaXial Crysof drawing out of the melt and holding a mon.. talline material which is particularly adapted to atomic 1ayer of atoms in a predetermined netproduction by means 0f the process of the preswork having the same geometry as, and Substanent invention is sodium nitrate, and Fig. 1 illustially the same size as, a like atomic network in trates apperaius found useful in Carrying out `the final formed crystaL solo appllcallon disthe process to produce a basal section of said closes the use of a cleavage section of mica as an 3o materiall- Contalflel' or holloW block lll in FigeXample of a particularly suitable material for 1 may comprise any suitable heat-insulating mainfluencing the growth of the new crystal. It is terial Such, for example as asbestos 0r the Inafurther disclosed ln said applloatlon that the terial sold under the trade name Marmite mico may .be floated on the @op of the melt and Within container or hollow block lil there is the new crystal grown from the top down, or that provided a cavity I2 which may be circular or the mica may be positioned at tno bottom of the of any other desired Shane Cup I4 Within cavmeit and the melt crystaiiized from the bottom. ity 2 is illustrated as Substantially oiled with Excellent results have been obtained with the molten Sodium nitrate l5- Cup i4 may beformed :process shown in said application from the standfrom a variety of materials, but preferred results point of producing large basal sections of sodium have been obtained With aluminum foil alJDrOX- 'nitrate and mixed crystals containing sodium imately 0.0015-O-003 inch in thickness. Other manitrate. In practicing said process for quantity terials may be used provided however that lf the production, however, it has been found that there materiel be one lio Wllloh Sodium nitrate will adls a tendency ln the new crystals for the optic here strictly on solidifying, said material` either aXis to fail to remain perpendicular to a plane should have substantially the same coflicient of over the whole area of the crystal but rather to thermal expansion as Solid Sodium nitrate 01' wander so as to be substantially perpendicular Should be refifllly deformable by the 'Sodium nito an approximately spherical or Cylindrical surtrate as the latter cools after crystallization. On face of a relatively large radius of curvature, said the other hand' other rigid `materials may be surface being convex on the side of the crystal used PTOVlded the adhesion thereto of 1ille Sodium adjacent the mica, This in turn Creates probnitrate is not suliicient to crack or strain the lems of parallax when the crystal is viewed in crystal unduly on coolingl polarized light. For example, if such' a basal In Fig. 1 it will be noted that an annular space section is mounted between circular polarizers i6 is provided between the outside of cup I4 and there is visible therein an interference pattern the walls. This arrangement is not essential but iS Preferred from the standpoint of ease of handling. Element I8 in Fig. l represents a cleavage section of mica floating on melt I and preferably at least sulciently smaller in area than cup Ill to fit readily therein. Element 20 in Fig. l is a thin plateV of rigid material inert with respect'to melt I5. Preferred examples of such materialinclude glass and mica, but other materials may be used provided they have melting points sufficiently higher than that of the melt to insure their remaining rigid at the temperatures utilized in the practice of the invention. The improved results obtained by means of the invention appear to derive primarily from use of element 2G, as will loe explained hereinafter.

In the practice of the invention with the equipment shown in Fig. l, cup I4 may be lled with ground sodium nitrate and both the hollow block and cup then heated until the charge is completely melted. Preferred results have been obtained by placing both hollow block and cup in a suitable oven ysuch as an electric muilie oven during this step. Alternatively, the sodium nitrate may be melted separately and then poured into cup le, but in either case it is desirable to heat both the melt and hollow block I0 substantially above the melting point of the charge, as, for example, by the use of an oven heated to an interior temperature of the order of 70D-750 F. The next step is to remove the hollow block from the oven, insert plate 20 in the melt and force it to the bottom thereof, and oat mica I8 on top of the melt. This operation will he found relatively simple, since mica is not readily wetted by the melt and will therefore float thereon in 'spite of its greater density. On the other hand, it will readily sink to the bottom there-of if the top is wetted. Care should be taken, however, to prevent trapping air bubbles under either of elements I8 and 20.

The next step of the process is preferably to return lthe hollow block to the oven for a short heating period, for example minutes, in order tocounteract the effects of the cooling while the mica was added and to establish as nearly as possible a uniform temperature throughout the melt. Thereafter the hollow block and melt may .be permitted to cool and the rate of cooling shouldV initially be controlled relatively carefully. For example, satisfactory results have been obtained by controlling the cooling to a cycle of from 2-21/2 hours to a temperature of the orderv of 10Q-450 F. During this cooling period, it is imp-ortant to insure that the melt cool fastest from the upper surface in order that crystallization may be initiated on mica I8 and in order to prevent the initiation of crystallization elsewhere in the melt. In other words, the thermal gradient in the melt should be maintained from the top to the bottom thereof. It has been found that such control is obtained by following the above procedure, and it may also be desirable to provide the hollow block with an additional cover, such as a sheet 22 of mica or aluminum, to protect the melt from any air currents.

As already indicated, it is preferable to permit the hollow block to remain within the oven until the temperature has fallen to near 400 F., but thereafter it may be removed to conditionsv of room temperature at any desired time without any appreciable diiference in the results. Similarly, cup M maybe removed from the solidified crystal at any desired point after the hollow block istaken from the oven, as by peeling in the case 4 of the aluminum foil indicated as a preferred material. Mica I8 may be separated from the solidified crystal with comparative ease at room temperature, although it is desirable to aid the separation with a drop oi water in order to minimize the danger of stresses producing either cleavage or regions of glide twinning'. Plate 2li may be similarly removed at the same time.

When the above steps have been carried out, the product will have substantially the appearance indicated in Fig. 2 and will comprise what may be considered as two different portions. Portion 30 will be by far the larger and will be a single crystal having its optic axis uniformly disposed perpendicular to the surface which was in contact with mica I8 as is indicated by arrow 32. This differs from the results frequently obtained when plate is not used in that in the latter case the optic axistends to be disposed in the manner indicated by the dotted arrows 33.

The other portion 34 in Fig.A 2 will comprise -whatf may be termed a species of undergrowth comprising randomly oriented sodium nitrate.: The relative thicknesses ofportions 30 and 34 will in duce crystallization only atthe surface of mica I8; it is possible to avoid the formation of; portion 34 virtually completely. On the other hand, a small amount of this randomly orientedundergrowth is not disadvantageous since if it is of appreciable thickness it will be found to part readily from portion 3l) along a well-defined- 35 plane 35, and if the undergrowth is thin, it can readily be removed by polishing as, for example,

with a belt moistened with water.

to any desired iinal thickness and size.

The, process of the present invention is not have been obtained with mica.`

pansion in the plane of contact therebetween.

Examples of other crystals lwhich' may be grown by the process of the invention include mixtures of sodium nitrate with, for example, silver., nitrate.

compounds, such as alkali halides. The present invention is equally applicable to said materials but since they are isotropic the problemof convexity, or non-uniform disposition of the opticE axis, does not arise.

Since certain changes in carrying out the above..

process may be made without departingA from its scope, it isintended that all matter containedl in the above description or shown in the accom'- panying drawing shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all thelgeneric and.,- specificA featuresV of the invention'l herein .de-V.

scribed, and all statements of the scope of the general reflect the rate of cooling during theV period vof crystallization. If the thermal gradlents in the melt are carefully controlled to pro-` Y Thereafter, crystal portion 30 may be ground and polished..

The above-identified joint applicationalso discloses the use ofemica in the growing` 60 of large, single crystals of l other alkali metal f invention which, as Va matter of language, might besaiclto fall therebetween. g 4

fWhatisclaimedfis: f 1.I Infaf-processv of producing a single uniaxial 'crystal having a predeterminedly oriented axis, the steps comprising forming Vin an open vessel a melt comprising `said crystalline material, positioning at the` bottom of said melt a relatively -thin lelement comprising la rigid material inert t with respect to said melt and having a substan- -tially highermelting point than said crystalline material, bringing into contact-With'the upper 1 surface of said melt a single crystal of ama- 1terial Asubstantially infusible at the temperature l"ofsaid melt, substantially insoluble in said melt, separable fromaiormed ycrystal and :diierent than said meltf'said second-named crystal' having a plane surface thereof'incontact with said melt and'havin'gsaid surface extending over at least a greater part o f the` cross-sectional-area of the melt'fsaid surface'-substantiallydening the de Asire'd"'cross-sectional area of the said crystal to b e 'formedfand initiating crystallization of said "melt on said crystal surface by cooling the surace of said melt ata faster rate than the remainde thereof. 'l

@2; In a-fprocess Aof producing a" single uniaxial crystal having a predeterminedlyoriented axis, thesteps comprisingforming in an open vessel a melt comprisingsaid crystalline material, positioning at the bottom of said melt a relatively thin element comprising a rigid material inertV with respect to said melt and having a substantially higher melting point than said crystalline material, bringing into Contact with the-upper surface of said melt a single crystal of a material substantially infusible at the temperature of said melt, substantially insoluble in said melt, separable from a formed crystal and different than said melt, said second-named crystal having a plane surface thereof in contact with said melt and having said surface extending over at least a greater part of the cross-sectional area of the melt, said surface substantially defining the desired cross-sectional area of the said crystal to be formed, initiating crystallization of said melt on said crystal surface by cooling the surface of said melt at a faster rate than the remainder thereof, and continuing crystallization of said melt on said crystal surface by maintaining a thermal gradient in said melt from the top to the bottom thereof.

3. In a process of producing a single uniaxial crystal having a predeterminedly oriented axis,

the steps comprising forming in an open vessel comprising aluminum foil a melt comprising said crystalline material, positioning at the bottom of said melt a relatively thin element comprising a rigid material inert with respect to said melt and having a substantially higher melting point than said crystalline material, bringing into contact with the upper surface of said melt a single crystal of a material substantially infusible at the temperature of said melt, substantially insoluble in said melt, separable from aV formed crystal and different than said melt, said secondnamed crystal having a plane surface thereof in contact With said melt and having said surface extending over at least a greater part of the cross-sectional area of the melt, said surface substantially defining the desired cross-sectional area of the said crystal to be formed, initiating crystallization of said melt on said crystal surface by cooling the surface of said melt at a faster rate than the remainder thereof, and continuing crystallization of said melton said crystal surface by maintaining a thermal gradient in said melt from the top Vto the bottom thereof.

4. In a process of producing a predeterminedly oriented section of a uniaxial crystal comprising sodium nitrate, the steps comprising forming in an open vessel a melt comprising said sodiumnitrate, positioning at thebottom of said melt a relatively thin. element comprising a rigid material inert with respect to lsaid melt andi having a substantially higher 'meltingpoint `thansaid crystalline material, bringing into contact with the upper sufac'e of said melt a single lcrystal of a 'materialsubstantially infusible at the temperature of said melt, substantially insoluble in said melt, separable from a formed crystal and different than said melt, said second-named crystalhaving a plane surface thereof in. contact with said melt and havingsaid surface extending over at least a, greater part of the crosssectional area of the melt,l said surface substantially defining the desired cross-sectional area of the said crystal to be formed, initiating crystallization of said melt on said crystal surface by cooling the surface of said melt at a faster rate than the remainder thereof, and continuing crystallization of said melt on said crystal surface by maintaining a thermal gradient in said-melt from the top to the bottom thereof.

5. In a process of producing a single uniaxial .crystal having a predeterminedly oriented axis, the steps comprising forming in an open vesse1 a melt comprising said crystalline material, positioning at-the `bottom of said melt a relatively thin element comprising a rigid material inert with respect to said melt and having a substantially higher melting point than said crystalline material, bringing into contact withV the upper surface of said melt a cleavage surfaceof mica, and initiating crystallization of said melt on said cleavage surface by cooling the surface of said melt at a faster rate than the remainder thereof.

6. In a process of producing a single uniaxial crystal having a predeterminedly oriented axis, the steps comprising forming in an open vessel a melt comprising said crystalline material,` positioning at thebottom of said melt a relatively thin element comprising a rigid material inert with respect to said melt and having a substantially higher melting point than said crystalline material, bringing into contact with the upper surface of said melt a cleavage surface of mica, initiating crystallization of said melt on said cleavage surface by cooling the surface of said melt at a faster rate than the remainder thereof, and continuing crystallization of said vmelt on said crystal surface by maintaining a thermal gradient in said melt from the top to the bottom thereof.

7. In a process of producing a predeterminedly oriented section of a uniaxial crystal comprising sodium nitrate, the steps comprising forming in an open vessel a melt comprising sodium nitrate, positioning at the lbottom of said melt a relatively thin element comprising a rigid material inert with respect to said melt and having a substantially higher melting point than sodium nitrate, floating on top of said melt a cleavage section of mica, and initiating crystallization of said melt on said mica by cooling the surface of said melt at a faster rate than the remainder thereof.

8. In a process of producing a predeterminedly oriented section of a uniaxial crystal comprising sodium nitrate, the steps comprising forming in an open vesse1 a. melt comprising sodium nitrate,

Certificate of Correction Patent No. 2,414,679 January 21, 1947 CUTLER D. WEST It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 2, line 45, for cocient read eoecz'ent; column 6, line 57, for crystal read cleavage;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 4th day of July, A. D. 1950.

[SEAL] THOMAS F. MURPHY,

Assistant Oommz'ssz'oner of Patents. 

